Best prog for making a help file?

I'm in charge of making a handbook for some software and I'm not sure how to do it. Previously we've converted .doc files into .pdfs so that the user could simply read it but it seems stupid that we're not taking advantage of linking things throughout the document etc. Anyway, my question is: What program (if I even need one) is best for creating a help file?

As much as I hated it microsoft compiled help is pretty much the standard for internal help documents. It's basicallly HTML and is easy to create just get the editor directly from MS. It makes a help document that looks like the ones almost all programs use when you go to Help->Contents